This invention relates to a process for producing microporous poly (tetrafluoroethylene- co-perfluoro (alkyl vinyl ether)) (PFA) or poly (tetrafluoroethylene-co-hexafluoropropylene)(FEP) membranes and to the membrane so-produced.
Microporous membranes include thin sheets and hollow fibers generally formed from synthetic thermoplastic materials and having a substantially continuous matrix structure containing open pores or conduits of small size. The mean pore size range for pores of "microporous membranes" is not precisely defined in the art, but it is generally understood to extend from about 0.05 microns to about 10 microns. Microporous membranes having open pores thereby importing permeability are useful in filtration. Microporous membranes having closed pores are impermeable. While not useful in filtration they are useful in other applications such as for thermal insulation.
PFA and FEP polymers are desirable filtration membrane materials because of their excellent chemical and thermal stabilities. However, their inherent inert nature also renders them unamenable to be cast into membranes by conventional solution immersion casting processes. Currently, microporous membrane using similarly inert material is disclosed in U.S. Pat. Nos. 3,953,566; 3,962,153; 4,096,227; 4,110,392 and 4,187,390. The process disclosed in these patents comprises stretching sintered poly(tetrafluoroethylene) (PTFE) particles to create a pore structure characterized by nodes interconnected by fibrils. The pores are highly elongated in the stretch direction.
U.S. Pat. Nos. 4,623,670 and 4,702,836 disclose a process for forming microporous membranes from fluoropolymer resins selected from the group consisting of ethylene-tetrafluoroethylene copolymer, ethylene-chlorotrifluoroethylene copolymer and poly(chlorotrifluoroethylene). In this process, an inorganic filler is required in melt molding the polymer with a chlorotrifluoroethylene oligomer. The filler and oligomer are dissolved out of the polymer to form voids. The use of fillers in microporous membranes used in filtration is highly undesirable since all of the filler cannot be removed by solvation and the filler remaining may migrate into the filtrate and contaminate it. Although the three fluoropolymers disclosed by these patents have good chemical and thermal resistance, they are inferior in stabilities when compared to PFA and FEP.
A method for making a porous fluorinated polymer structure is disclosed in U.S. Pat. No. 4,434,116. It involves forming a solvated or partially solvated polymer/solvent mixture. The polymer for which this is applicable comprises a copolymer of tetrafluoroethylene and perfluoro vinyl ether with a sulfonyl fluoride (--SO.sub.2 F), sulfonate (--SO.sub.3 Z) or carboxylate (--COOZ) functional group wherein Z is a cation. The presence of the polar functional group greatly enhances the dissolution of this polymer. A variety of organic solvents have been reported. The method described is based on thermal phase separation of the polymer/solvent mixture and specified that the solvent (porogen) is a solid at room temperature and must crystallize after phase separation. The solvent is then removed from the blend in the solid state. No pore morphology or permeability data of the porous structure were given.
It would be desirable to provide a versatile process for producing microporous membranes from PFA or FEP which permits the use of liquid solvent to form a solution at elevated temperature which undergoes a liquid-liquid or a solid (crystalline)-liquid phase separation upon cooling where the polymer is the solid phase. In addition, it would be desirable to provide such a process in which the pore structure can be well controlled. In addition, it is advantageous to eliminate the need for stretching of the product during formation so that the product can be deposited directly upon a suitable substrate such as a porous woven or nonwoven substrate which cannot be stretched. If such a process were available, the membrane could be formed onto the porous substrate directly in order to produce laminates having increased mechanical strength which would permit their use in a wide variety of filtration environments.